Class 12 Chemistry Chapter 10 Biomolecules

Key Concept: Monosaccharide Examples

c) Fructose
[Solution Description] Fructose is an example of a ketose sugar, which contains a keto group.

Key Concept: Preparation Methods

c) Hydrolysis with dilute $H_2SO_4$
[Solution Description] Glucose can be prepared from starch by hydrolysis. The process involves boiling starch with dilute $H_2SO_4$ at 393 K under pressure, which breaks down starch into glucose units.

Key Concept: DNA vs RNA Bases

c) Thymine
[Solution Description] DNA contains adenine, guanine, cytosine, and thymine. RNA has uracil instead of thymine. Therefore, the base present in DNA but absent in RNA is thymine.

Key Concept: Nucleotide Components

b) Deoxyribose
[Solution Description] The sugar present in DNA is deoxyribose, which is a five-carbon sugar. This distinguishes DNA from RNA, which contains ribose.

Key Concept: Enzyme Quantity

b) Small quantities

[Solution Description] Enzymes are highly efficient catalysts, meaning they are active at even very low concentrations. Thus, they are only needed in small quantities to effectively catalyze reactions.

Key Concept: Nucleoside Definition

b) Nucleoside
[Solution Description]
A nucleoside consists of a nitrogenous base attached to the 1′ position of a pentose sugar. This does not include any phosphate group.

Key Concept: Nucleotide Polymerization, DNA Structure

d) Their 5′ to 3′ directionality is opposite.

[Solution Description]

The antiparallel nature of DNA strands refers to the orientation of the two strands that run in opposite directions. This is due to the fact that nucleotides within a single strand are connected by phosphodiester bonds between the 3′ carbon of one sugar and the 5′ carbon of the next sugar. Thus, if one strand runs from 5′ to 3′, the complementary strand must run from 3′ to 5′. This arrangement maximizes hydrogen bonding between complementary bases (adenine pairs with thymine via two hydrogen bonds, and guanine pairs with cytosine via three hydrogen bonds), stabilizing the double helical structure.

Key Concept: Vitamin Function

d) Vitamin D
[Solution Description] Vitamin D plays a crucial role in regulating calcium and phosphate metabolism by promoting their absorption from the gut and maintaining appropriate serum concentrations for normal bone mineralization.

Key Concept: Essential vs Non-Essential, Real-World Application

b) Lysine
[Solution Description] Essential amino acids cannot be synthesized by the body and must be obtained from the diet. In vegetarians, lysine is often less abundant because it is prevalent in meat but not in most plant proteins. Lysine is crucial for protein synthesis and energy metabolism. If deficient, one may experience fatigue and poor muscle recovery due to impaired protein production and repair processes.

Key Concept: Basic Definition

d) Minerals
[Solution Description] Biomolecules are chemical compounds found in living organisms, typically including carbohydrates, proteins, nucleic acids, and lipids. Minerals are inorganic substances that do not fit into this category.

Key Concept: Nucleotide Components

b) Base, sugar, phosphate
[Solution Description] A nucleotide is composed of three main components: a nitrogenous base, a pentose sugar, and a phosphate group. This complete structure forms the building blocks of nucleic acids such as DNA and RNA.

Key Concept: Nucleic Acid Types

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description]

The assertion is true because mRNA, a type of RNA, carries the genetic code from DNA to the ribosome for protein synthesis. However, the reason given is not incorrect, as tRNA indeed carries amino acids to the ribosome during protein synthesis, but it does not directly explain why RNA is responsible for carrying the genetic code from DNA. Thus, both statements are true, but the reason is not the correct explanation for the assertion.

Key Concept: Solubility Classification

c) Vitamin E
[Solution Description] Fat-soluble vitamins are stored in the liver and adipose tissues. They include Vitamin A, D, E, and K.

Key Concept: Reducing Sugar Identification

d) Glucose
[Solution Description] Reducing sugars are those that can donate electrons to another chemical, like Fehling’s solution or Tollens’ reagent. Glucose, being an aldohexose with a free aldehyde group in its open-chain form, acts as a reducing agent. It can reduce both Fehling’s and Tollens’ solutions, identifying it as a reducing sugar.

Key Concept: Structural Differences

d) Glycogen’s extensive branching allows for quick enzyme access
[Solution Description] Glycogen is more branched than amylose, allowing for multiple sites of enzymatic action, which facilitates quicker glucose release when needed.

Key Concept: Genetic Information Storage and Transfer

c) Assertion is true, but Reason is false.

[Solution Description]

The assertion that DNA stores genetic information in the form of a sequence of nucleotides is true. DNA consists of sequences of four types of nucleotides, with their order encoding the genetic instructions needed for the development, functioning, growth, and reproduction of all known organisms.

The reason given states that RNA is responsible for storing genetic information permanently, which is false. RNA does not store genetic information permanently; instead, it plays a role in the expression of genes by acting as an intermediary between DNA and proteins during transcription and translation. Thus, while both statements involve nucleic acids, the Reason does not support the Assertion correctly.

Therefore, the correct answer is: (c) Assertion is true, but Reason is false.

Key Concept: Application in Proteins

b) $\beta$-Pleated Sheet
[Solution Description] Fibrous proteins often contain more $\beta$-pleated sheet structures. These structures provide strength and rigidity due to their extended and aligned configuration stabilized by hydrogen bonds, making them suitable for structural roles in tissues like skin and hair.

Key Concept: Reducing vs Non-reducing

b) Maltose
[Solution Description] A reducing sugar has a free aldehydic or ketonic group that can reduce certain chemical reagents. Among the options, maltose and lactose are reducing sugars because they have free aldehydic groups after the glycosidic bond cleavage, whereas sucrose does not.

Key Concept: Real-World Denaturation, Protein Classification

c) Secondary and tertiary structures, losing typical classification
[Solution Description] Cooking meat results in denaturation, where heat breaks down hydrogen bonds, disulphide linkages, and other non-covalent interactions, leading to unraveling of secondary and tertiary structures. However, the primary structure remains intact. The proteins originally present, such as myoglobin (a globular protein), lose their native configuration and form a denatured state that doesn’t fit traditional fibrous or globular classifications. Therefore, the correct option is that secondary and tertiary structures are affected, but not primary.

Key Concept: Synthesis Pathways, Real-World Application

c) Transamination
[Solution Description] Essential amino acids must be obtained from dietary sources because the body cannot synthesize them. Non-essential amino acids can be synthesized within the body often using essential amino acids as precursors. The key mechanism in converting one amino acid type into another is transamination, where an amino group is transferred from an amino acid to a keto acid.

Key Concept: Linkage Identification

a) $\alpha$-1,4-glycosidic
[Solution Description] Maltose is a disaccharide formed by two glucose units linked via an $\alpha$-1,4-glycosidic bond. Each glucose unit contributes to forming the linkage, resulting in one oxygen bridge between carbon 1 of one glucose and carbon 4 of the other.

Key Concept: Fibrous vs. Globular Proteins

c) They possess fiber-like structures and are generally insoluble in water.
[Solution Description] Fibrous proteins generally have a long, fiber-like structure and are insoluble in water due to their alignment and types of bonding. In contrast, globular proteins are soluble and have a spherical shape. This structural difference contributes to their different roles in biological systems.

Key Concept: RNA Types

c) 3
[Solution Description] There are three main types of RNA involved in protein synthesis: mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).

Key Concept: Enzyme Quantity

b) They are reusable and not consumed in the reaction
[Solution Description] Enzymes are catalysts and are not consumed in the reaction process. Therefore, they can be reused multiple times, which means they are needed only in small quantities.

Key Concept: Reducing vs Non-reducing

b) Glucose
[Solution Description] Reducing sugars are those that can donate electrons to reduce other molecules, commonly identified by their ability to reduce agents like Fehling’s solution. All monosaccharides are considered reducing sugars due to their free aldehyde or ketone groups. Among the given options, glucose has a free aldehyde group, thus classifying it as a reducing sugar.

Key Concept: Strand Structure

a) Single strand

[Solution Description] RNA is typically single-stranded, as opposed to DNA, which is double-stranded.

Key Concept: Enzyme Activation Energy

b) 4.07 kJ mol$^{-1}$
[Solution Description]
Sucrase reduces the activation energy for the hydrolysis of sucrose from 6.22 kJ mol$^{-1}$ to 2.15 kJ mol$^{-1}$. To find the reduction in activation energy, we subtract the lower activation energy from the original one:
$$\text{Reduction} = 6.22 \, \text{kJ mol}^{-1} – 2.15 \, \text{kJ mol}^{-1} = 4.07 \, \text{kJ mol}^{-1}.$$
Therefore, the activation energy is reduced by 4.07 kJ mol$^{-1}$.

Key Concept: Real-world Application

b) Due to protein denaturation caused by heat
[Solution Description] Boiling an egg causes the proteins in the egg white to denature due to the high temperature. This process involves breaking hydrogen bonds within the protein structure, causing them to unfold and form new bonds with each other, resulting in a solid, coagulated structure.

Key Concept: Hormone Classification

b) Testosterone
[Solution Description] A steroid hormone is typically produced by the adrenal cortex and gonads. Testosterone fits this category as it is responsible for secondary male characteristics and is a well-known steroid hormone.

Key Concept: Hormonal Influence on Metabolism, Hormone Receptor Interactions

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description]

The assertion states that glucocorticoids have a dual role: they promote gluconeogenesis to increase blood glucose levels and simultaneously suppress the immune system, which also reduces inflammation. This is true as glucocorticoids like cortisol are known for these actions. They help the body respond to stress by ensuring an adequate supply of glucose while also modulating immune responses.

The reason claims that glucocorticoids achieve their effects exclusively by binding to cytoplasmic receptors, which then alter gene expression. While it is true that glucocorticoids bind to intracellular (cytoplasmic) receptors, this description is incomplete because the subsequent translocation of the receptor-hormone complex into the nucleus affects transcriptional activity, thereby regulating various physiological processes including metabolism and immune function. However, the reason provided does not capture the full complexity of glucocorticoid action beyond metabolism to include other systemic effects like immune suppression.

Therefore, both Assertion and Reason are true; however, the Reason does not fully explain all aspects of the Assertion, particularly the immune suppression factor.

Hence, the correct option is b).

Key Concept: Comparative Analysis, Structural Analysis

d) Both, because they have free aldehydic groups
[Solution Description]
Both maltose and lactose are disaccharides that exhibit reducing properties due to the presence of a free aldehydic group in at least one of their monosaccharide components.

Maltose consists of two glucose units linked via an $\alpha$-1,4-glycosidic bond. One of the glucose residues retains a free aldehyde group at C1, making maltose a reducing sugar. Lactose, on the other hand, comprises galactose and glucose joined by a $\beta$-1,4-glycosidic bond. Similarly, the glucose moiety has a free aldehydic group at C1, which renders lactose a reducing sugar as well.

Therefore, both sugars can reduce other substances due to this structural feature.

Key Concept: Basic Definition

c) To be a major energy source
[Solution Description] Carbohydrates primarily serve as an energy source for living organisms. They are broken down to release energy used by the body to perform various functions.

Key Concept: Gender-Specific Hormones

c) Estradiol
[Solution Description] Estradiol is responsible for the development of secondary female characteristics as well as controlling the menstrual cycle.

Key Concept: Linkage and Optical Activity, Linkage and Functional Groups

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description] Sucrose is a non-reducing sugar composed of $\alpha$-D-glucose and $\beta$-D-fructose linked by their respective C1 and C2 atoms, making the glycosidic bond involve both anomeric carbons so that neither glucose nor fructose has any free aldehydic or ketonic group in sucrose. On hydrolysis, this linkage is broken, releasing separate monosaccharides: glucose and fructose. Glucose is dextrorotatory, whereas fructose is laevorotatory with an even greater rotational strength than glucose. Hence, the overall mixture becomes laevorotatory. This change in optical activity indeed happens because of breaking down into free sugars but not because of a change in reducing properties as postulated in the reason. Thus, both statements are true, but the reason provided does not correctly explain the assertion’s mechanism related to optical rotation alteration.

Key Concept: Nucleoside Definition

d) 1′
[Solution Description] A nucleoside consists of a nitrogenous base attached to the 1′ carbon atom of a pentose sugar.

Key Concept: Real-world Application, Complex Formation

b) By stabilizing the interactions between subunits
[Solution Description]
The catalytic efficiency of multi-subunit enzymes can be influenced by the stabilization or destabilization of their quaternary structures. In this case, ions could potentially enhance or stabilize interactions among the subunits such as hydrogen bonds or electrostatic interactions, thus facilitating better alignment and cooperation between the subunits. This leads to improved catalytic activity. Enhanced binding specificity or structural stability due to ionic interaction could be responsible for the observed increase in efficiency.

Key Concept: Sugar Type

a) Ribose
[Solution Description] Nucleic acids have a pentose sugar as part of their structure. DNA contains 2-deoxyribose, whereas RNA contains ribose. Thus, the sugar present in RNA is ribose.

Key Concept: Hormone Production Site

c) Gonads (testes)

[Solution Description] Testosterone is primarily produced in the gonads (testes in males). The Leydig cells located in the testes are responsible for this hormone’s production, which plays a major role in developing secondary sex characteristics in males.

Key Concept: Complex Interactions, Folding Pathways

b) Increased rigidity preventing fibrous extension
[Solution Description]
The folding initiation at the globular region can influence the overall protein conformation significantly. It could lead to stabilization that restricts the flexibility needed for the fibrous region to achieve its extended shape. As the globular region typically involves more complex intramolecular interactions such as hydrophobic core formation and precise side-chain packing, its early folding might restrain the fibrous section from unfolding into an elongated conformation, potentially affecting function.

Key Concept: Structural Comparison, Protein Denaturation

d) Repositioning of subunits in quaternary structure
[Solution Description]
Globular proteins’ enzymatic activities are highly dependent on their three-dimensional shapes, specifically the tertiary structure. Changes at the primary level alter the amino acid sequence, potentially changing active sites. Secondary structural alterations can also impact the overall shape. Tertiary structure changes directly affect the active site configuration. However, slight alterations at the quaternary level (like minor subunit repositioning) might not necessarily affect function unless they block or distort active sites.

Therefore, changes at the quaternary level are generally less impactful on enzymatic activity compared to other levels.

Key Concept: Solubility Classification

c) Vitamin E
[Solution Description]
Vitamins are classified based on their solubility into two categories: water-soluble and fat-soluble. Fat-soluble vitamins include Vitamin A, D, E, and K. Therefore, Vitamin E is a fat-soluble vitamin.

Key Concept: Function Recognition

b) Energy storage
[Solution Description] Glycogen serves as the main storage form of carbohydrates in animals. It is similar to amylopectin but more highly branched. Its primary function is to provide energy when needed by breaking down into glucose.

Key Concept: Mechanism Analysis, Advanced Application

c) Secondary and tertiary structures are destroyed but primary structure remains intact.
[Solution Description] As the temperature increases, proteins absorb heat energy, causing their molecular vibrations to increase. This disrupts weak interactions like hydrogen bonds and van der Waals forces that maintain secondary and tertiary structures, leading to denaturation. The primary structure (sequence of amino acids) remains unchanged. Hence, the loss of biological activity occurs along with the destruction of 3D conformation.

Key Concept: Hormone Balance

c) Assertion is true, but Reason is false.

[Solution Description] The assertion is true because mineralocorticoids, such as aldosterone, play a crucial role in maintaining electrolyte balance by promoting sodium retention and potassium excretion in the kidneys. This process is vital for controlling blood pressure and fluid balance in the body. However, the reason is false because glucocorticoids primarily regulate carbohydrate metabolism and stress responses rather than directly influencing salt and water excretion. Therefore, while the assertion is correct, the reason provided is incorrect.

Key Concept: Zwitter Ion Formation

c) Zwitter ion

[Solution Description] In an aqueous solution, amino acids can exist as zwitter ions. This occurs when the $-NH_2$ group accepts a proton, forming $-NH_3^+$, and the $-COOH$ group loses a proton, forming $-COO^-$. The molecule thus becomes a dipolar ion with both positive and negative charges.

Key Concept: Vitamin Storage Exception

c) Vitamin B12
[Solution Description] Among the water-soluble vitamins, Vitamin B12 is unique because it can be stored in the liver for several years, unlike others that need regular replenishment due to constant excretion in urine.

Key Concept: Enzyme Functionality

b) To prevent denaturation which affects their active sites.
[Solution Description] Enzymes are globular proteins sensitive to temperature and pH changes. They work most effectively under mild conditions found in living organisms because extreme temperatures or pH levels may denature them, altering their structure and function.

Key Concept: Vitamin Solubility

c) Vitamin D
[Solution Description] Water-soluble vitamins include vitamins from the B group such as B1, B6, and B12, along with Vitamin C. Fat-soluble vitamins are stored in the body’s fatty tissues and include Vitamins A, D, E, and K. Among the options given, Vitamin D is a fat-soluble vitamin and hence not water-soluble.

Key Concept: Double Helix Structure of DNA

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description]

The assertion correctly states that the two strands of DNA are oriented antiparallel to each other. In an antiparallel orientation, one strand runs 5′ to 3′ while the complementary strand runs 3′ to 5′. The reason given suggests that this orientation is responsible for base pair specificity. However, specificity arises from hydrogen bonding between specific bases (adenine with thymine, and cytosine with guanine), not directly due to the antiparallel orientation. Thus, both the assertion and reason are true, but the reason does not explain the assertion.

Key Concept: Enzyme Specificity Reason

d) Due to the shape of the enzyme’s active site
[Solution Description] Enzyme specificity is largely due to the complementary shape between the enzyme’s active site and the specific substrate molecules. This “lock and key” model ensures that only specific substrates can bind effectively to their respective enzymes.

Key Concept: Solubility and Storage

d) Vitamin C

[Solution Description] Water-soluble vitamins like Vitamin C are not stored in the body and require regular consumption through diet.

Key Concept: Zwitterion and Solubility, Amphoteric Nature and Biological Function

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

[Solution Description]

The assertion states that the amphoteric nature of amino acids accounts for their high solubility in water. While the amphoteric nature indicates they can react with both acids and bases, the key reason for their solubility is their zwitterionic nature.

Zwitterions have a dipolar ion form that interacts strongly with polar solvent molecules like water, enhancing solubility. Therefore, while the amphoteric property contributes to reactivity, it doesn’t directly relate to solubility.

For the reason, the zwitterionic form does indeed allow amino acids to react with both acids and bases, confirming its truth. However, this statement does not serve as an explanation for the assertion about solubility. Thus, the assertion and reason are true but not causally linked as stated.

Key Concept: Simple Reaction Outcome

b) Dipeptide
[Solution Description] When two amino acids react, they form a dipeptide. This involves the formation of a single peptide bond between them by eliminating a water molecule.